A TURTLE EGGSHELL FRAGMENT FROM THE EOCENE BRIDGER FORMATION

Abundant sauropsid eggshell fragments have been reported in lake margin and fluvial sediments associated with the large Eocene basins of Wyoming, Colorado and Utah. Previous descriptions of fossil eggshell fragments from the Green River, Wasatch, DeBeque and Bridger Formations are consistent with avian and crocodilian interpretations. In this report, a single turtle eggshell fragment is described from the Eocene Bridger Formation (Bridger A). The study site is located 10 km south of Opal, Wyoming.

The eggshell fragment was studied in radial cross-section with light microscopy and polarized light microscopy. The resultant micrographs were compared with published reports of modern and fossil “ornithoid”, “crocodiloid”, “testudoid” and “geckoid” eggshell. The Bridger Formation eggshell fragment shows the following “zones” of the basic shell unit: 1) an organic core and 2) aragonite radial ultrastructure. The eggshell fragment is 0.25 mm thick and the basic (vertical) shell units are about 0.1 mm wide. These eggshell characteristics are consistent with those of modern turtle eggshell.

The turtle eggshell fragment was used in the construction of a single fossil caddisfly pupal case. Other particles in the caddisfly case are ostracods and ooids. The caddisfly case was preserved in a microbial carbonate bed in Bridger A, about 20 m below the Lyman Limestone Marker Bed (G marker bed). At the study site the caddisfly-containing microbial carbonate bed is a 20 cm thick ostracodal oolitic sparite that can be used for regional correlation (E marker bed). Other fossils that occur at the study site include logs, stumps, fish bones and stromatolites. Toward basin center, the oolitic limestone grades into oil shale.

The caddisfly-containing microbial carbonates formed in high-energy nearshore areas of a large lake that expanded into forested areas. The lake was long-lived as evidenced by the incorporation of multiple generations of caddisfly pupal cases in the microlaminated microbial carbonate. The turtle eggshell fragment was likely derived from nearby subaerially exposed areas of the lake margin.